九州大学大学院総合理工学府報告第 41 巻 第 2 号 22-29 頁 令和 2 年 2 月

Engineering Sciences Reports, Kyushu University
Vol. 41, No. 2, pp. 22-29, FEB, 2020

Hydrogen Production from Formic Acid Decomposition
Using Au@Pd and Au@Pd/TiO2 Core−shell Nanocatalysts
Prepared by Microwave Heating
Masaharu TSUJI*1,2† Kento FUKUTOMI*3 Masashi HATTORI*1 Daisuke
SHIMAMOTO*3 Keiko UTO*1 Jun-Ichiro HAYASHI*1,2 and Takeshi TSUJI*4
E-mail of corresponding author: tsuji@cm.kyushu-u.ac.jp†(Received January 23, 2020, accepted January 30, 2020)
Formic acid (FA: HCOOH) has attracted attention as a liquid fuel for use in hydrogen fuel cells
because it has high energy density and it can be handled safely. For hydrogen production from FA
decomposition at low temperature, Au@Pd core–shell nanocatalysts supported on TiO2 nanoparticles
were prepared using microwave (MW) heating. The hydrogen production rates, 0.13, 0.21, and 3.2
L g-1 h-1, were obtained using Au@Pd/TiO2 at 30, 60, and 90 oC, respectively. These values were larger
than those obtained for bare Au@Pd catalysts by factors of 3.0, 1,8, and 1.5, respectively. Based on
negative chemical shifts of Pd peaks found in XPS data of Au@Pd/TiO2 particles in comparison withthose of bare Au@Pd ones, the enhancement of catalytic activity of Au@Pd particles in the presence ofTiO2 was explained by an electron-donating effect of TiO2 to Pd shells. This effect enhances theadsorption of formate to catalyst and dehydrogenation from formate leading to H2 + CO2.
Key words: Hydrogen production, Formic acid, Au@Pd core–shell nanocatalyst, TiO2 support,

Microwave heating

1.

Introduction

Hydrogen is a promising energy carrier thatcan be used to generate electricity in a fuel cellwith water being the only waste product. The
search for effective techniques of hydrogen gas
generation from liquid fuels has remained adifficult challenge for mobile hydrogen energy
systems. In this respect, formic acid (FA)attracts great attention as a liquid fuel because
it has high energy density, nontoxicity, andexcellent stability at room temperature. It is
known that Pd-based catalysts involving core–shell and alloy types of catalysts are effective
for the production of hydrogen from thedecomposition of FA at room temperature.1-8)
According to a systematic study on M@Pd (M =Ru, Rh, Pt, Ag, Au) core–shell nanocatalysts forthe production of hydrogen from the*1

Institute for Materials Chemistry and Engineering,and Research and Education Center of GreenTechnology*2 Department of Applied Science for Electronics andMaterials*3 Department of Applied Science for Electronics andMaterials, Graduate Student*4 Department of Materials Science, Shimane
University

decomposition of FA,9) which gave higher
catalytic activity than pure Pd/C catalysts, thebest catalytic activity has been obtained from
Ag@Pd catalysts, for which a high initialhydrogen rate of about 4 L g-1 h-1 was achievedat room temperature. The highest catalytic
activity of Ag@Pd core–shell nanocatalysts was
explained by the largest electron transfer fromthe Ag core to the Pd shell because of the lowest
work function of Ag (4.7 eV) among M
compared with that of Pd (5.1 eV).
To enhance the catalytic activity, we haverecently studied the preparation of Ag@Pd
nanocatalysts loaded on TiO2 nanoparticles
using a microwave (MW) heating in ethylene
glycol (EG) and aqueous solution,10-13) wherein
EG and polyvinylpyrrolidone (PVP) were used
as reductants.14,15) An advantage of MW
heating in comparison with conventional oilbath heating is strong adhesion of Ag@Pd
catalysts on TiO2 support. Based on sphericalaberration-corrected scanning transmission
electron microscopy (Cs-corrected STEM),
STEM-energy dispersed X-ray spectroscopy(EDS), X-ray diffraction (XRD), and X-ray
photoelectron spectroscopy (XPS), it could be
noted that the product nanocatalysts consisted

令和元年度

九州大学大学院総合理工学報告

of an Ag–Pd alloy core and Pd shell
nanocrystals loaded on anatase type TiO2
nanoparticles. Ag and Pd atoms were partiallyalloyed with each other under MW heating in
EG or H2O. Therefore, Ag93Pd7 alloy core andPd shell catalysts were formed in H2O solution
even at a short irradiation time (30 min). Usinga TiO2 support, about 13 times higher hydrogenproduction rate of 46.032.2 L g-1 h-1 at 30 oC
was obtained than that attained without a TiO2
support (3.67 L g-1 h-1 at 20 oC).9) The significant
enhancementofcatalytic
activityof
Ag93Pd7@Pd in the presence of TiO2 was
explained by further electron transfer from
TiO2 to Pd because the work function of TiO2
(4.0 eV) is lower than that of Ag (4.7 eV) and Pd(5.1 eV).
In this study, FA decomposition was studied
using bare Au@Pd and Au@Pd/TiO2 catalysts to
examine the effects of core-metal M and thepresence of TiO2 support in the M@Pd catalysts.
Catalytic activity was examined at 30, 60, and90 oC to determine activation energies of
Au@Pd and Au@Pd/TiO2 catalysts for FA
decomposition. The results obtained arecompared with those of AgPd@Pd and
AgPd@Pd/TiO2 catalysts reported previously.1013)

2.

Experimental

2.1.

Materials

HAuCl44H2O (>99.0%), sodium citrate(Na3C6H5O7·2H2O), ethanol (>99.5%), and PVP(average molecular weight of 10 k in terms ofmonomer units) were obtained from Kishida
Chem. Co., Ltd. Na2PdCl4 (>98.0%) was
purchased from Sigma Aldrich. Titanium
tetraisopropoxide (Ti(Oi-Pr)4) was obtained
from Wako Pure Chem. Ind., Ltd. and FA(>98%) was purchased from Kanto Chem. Co.,
Inc. 1,5-pentanediol (>97%) was obtained from
Tokyo Chem. Ind. Co., Ltd. All reagents wereused without further purification.
2.2

Preparation of TiO2 nanoparticles

Fig. 1 shows an experimental apparatus used
for the MW heating (Shikoku Keisoku Kogyo
K.K.;  type, maximum output power 750 W). A
three-necked glass flask was placed in the MWoven and a mixture of metallic reagent andsurfactant was added into the flask. A reflex
tube and a thermocouple were installed into a
MW oven through holes in the top. A
thermocouple made of an optical fiber, which is
not damaged under MW irradiation, was used.

第 41 巻第 2 号

23

Fig. 1. Microwave reactor used for the
preparation of Au@Pd/TiO2 catalysts.
When TiO2 nanoparticles were prepared by
MW heating, 3 mmol Ti(Oi-Pr)4 was added to
50 ml of 1,5-pentanediol.16) The solution wasirradiated in the MW oven at 200 W for 3 min.
Then, 2 ml of distilled water was added to thissolution and MW was irradiated again at 700
W for 1 h. The solution temperature increased
to 175 oC after 3 min heating at 200 W. It
dropped to 150 oC after injection of distilledwater and increased again to about 195 oC after
6 min at 700 W and became nearly constantafter that. For the use of TiO2 as a support for
metallic nanoparticles in the next step,
products were obtained by centrifuging thecolloidal solution at 15,000 rpm for 30 min twotimes. Then, they were redispersed in 50 ml ofdistilled water.
2.3

Preparation of bare Au@Pd nanocatalysts

Au@Pd core-shell nanoparticles were formedby using a two-step reduction method of
metallic salts. First, Au core nanoparticles
were formed by the following procedure. 45 mlof 2.22 mM HAuCl44H2O aqueous solution was
heating at 100 oC under conventional oil-bathheating. Then, 5 ml of 100 mM sodium citrateaqueous solution was added, and the reagent
solution was kept at 100 oC for 1 h. After that,
5 ml of 0.76 M PVP aqueous solution was added
as a protecting agent. The product solution wascentrifuged at 15,000 rpm for 100 min, andredispersed in distilled water. It was repeated
two times and finally dispersed in 12.8 ml ofdistilled water.
In the second step, Au@Pd nanoparticles
were prepared by MW heating of a mixture of
6.4 ml of Au core solution, 5 ml of 10 mM
Na2PdCl4 aqueous solution, 5 ml of 0.38 M PVP
aqueous solution, and 3.6 ml of distilled water.
The reagent solution was heated by MW

24

Hydrogen Production from Formic Acid Using Au@Pd and Au@Pd/TiO2 Nanocatalysts

irradiation at 400 W for 10 min under Ar gas
bubbling. The Ar gas was used to suppress
oxidation of Pd shell. The solution wascentrifuged at 12,000 rpm for 100 min, andredispersed in ethanol. This procedure wasrepeated three times. Similar centrifugal
separation was carried out three times indistilled water, and it was finally redispered in
10 ml of distilled water.
2.4
Preparation
nanocatalysts

of

Water

Au@Pd/TiO2

After mixing 6.4 ml of Au core solution with
3.6 ml of TiO2 solution involving 3.6 mmol TiO2
molecules under ultrasonic irradiation for 3 h,
5 ml of 10 mM Na2PdCl4 and 10 ml of 0.79 M
PVP aqueous solutions were added. Then, thereagent solution was heated by MW irradiation
at 400 W for 10 min under Ar gas bubbling. The
same centrifugal separation procedure as used
the preparation of Au@Pd nanoparticles wasused for the synthesis of 10 ml of aqueous
Au@Pd/TiO2 catalyst solution.
2.5

Gas buretteCharacterization of product particles

For TEM and TEM-EDS (JEM-2100F; JEOL)
observations, samples were prepared bydropping colloidal solutions of the products
onto Cu grids. The average sizes of productparticles were determined by measuring morethan 100 particles in TEM images. The XPS
spectra of the product were measured using Al
Ka radiation (AXIS-165; Shimadzu Corp.).

Glass tube
Catalysts +formic acid

Fig. 2. Gas burette used for the hydrogenproduction from decomposition of formic acidusing Au@Pd and Au@Pd/TiO2 catalysts.
generated gas (H2 + CO2), when xa reaches 20%,
mmetal is the weight of the metallic catalyst, andt is the reaction time when xa reaches 20%.

3.

Results and discussion

3.1 TEM and TEM-EDS images of Au@Pd and
Au@Pd/TiO2 catalysts
Figs. 3a and 3b show TEM image and XRD
pattern of TiO2 particles, respectively. Results
show that TiO2 support prepared by MW
heating was solely anatase-type with an

2.6 Hydrogen generation activity of Au@Pd
and Au@Pd//TiO2 nanocatalysts
Hydrogenproduction
rate
from
decomposition of FA was measured using theapparatus shown in Fig. 2. The hydrogenproduction activity of the prepared samples
was examined using the following method thetotal gas volume from a stirred glass tube
containing 20 ml of 0.25 M FA aqueous solution
and the prepared sample (metallic catalyst
weight of 5.8 mg) was measured using a gas
burette.
The hydrogen gas volume produced per gramof Au@Pd and Au@Pd/TiO2 catalysts per hour
was calculated using Eq. (2).
HCOOH  H2 + CO2

Rinitial = Vgas /(2 mmetal t)

(1)
(2)

In this equation, Rinitial represents the initialrate of hydrogen generation when conversion xa
reaches 20%, Vgas is the volume of the initial

Fig. 3. (a) TEM image and (b) XRD patternof TiO2 prepared by MW heating.

令和元年度

九州大学大学院総合理工学報告

average diameter of 102 nm. Fig. 4 shows atypical TEM image of Au particles prepared byoil-bath heating, where spherical nanoparticles
with an average diameter of 10±2 nm areobserved.

第 41 巻

第 2 号

Figs. 5 and 6 displays TEM and TEM–EDS
data of Au@Pd and Au@Pd/TiO2 catalysts
prepared by MW heating, respectively. On thebasis of these data, uniform Pd shells with anaverage thickness of 3±1 nm are formed on Au
cores and Au@Pd catalysts with an average
diameter of 15±2 nm were loaded uniformly on
TiO2 nanoparticles. The Au : Pd atomic ratio in
Au@Pd catalysts was estimated to be about 7 :3, respectively, from TEM-EDS data.

Fig. 4. TEM image of Au nanoparticles
prepared by oil-bath heating.

Fig. 5. (a)-(d) TEM and TEM-EDS images,(e) line analysis, and (f) EDS of Au@Pd
nanoparticles.

25

Fig. 6. (a)-(e) TEM and TEM-EDS images,(f) line analysis, and (g) EDS of Au@Pd/TiO2
nanoparticles.

26

Hydrogen Production from Formic Acid Using Au@Pd and Au@Pd/TiO2 Nanocatalysts

3.2 XPS spectra of Au@Pd and Au@Pd/TiO2
catalysts

3.3 Initial H2 production rates from formic
acid decomposition using Au@Pd and
Au@Pd/TiO2 catalysts
The initial H2 production rates (Rinitial) of
Au@Pd and Au@Pd/TiO2 catalysts from FA
decomposition were measured using a gas
burette (see Fig. 2). Detailed gas analyses for
CO2, H2, and CO were performed on a gas
chromatograph and no CO emission was
detected using GC for all samples at 30−90 ○C.
This implies that the following decomposition
channel is negligible in our conditions.
HCOOH  CO + H2O(3)

Intensity (Arb. Unit)

(a-1)
Au@Pd

88

(b-1)
Au@Pd

4f5/2

4f7/2

83.3 eV868482
Binding energy (eV)

340

80

(b) Pd335.1 eV

3d3/2

(b-2)
Au@Pd/TiO2345(a) Au83.3 eV(a-2)
84.0 eV
Au@Pd/TiO2

90

Intensity (Arb. Unit)

To characterize chemical states of Au@Pd
and Au@Pd/TiO2 catalysts, XPS spectra were
measured. In the range of 80–90 eV (Fig. 7a),
Au 4f5/2 and 4f7/2 peaks are observed. The
binding energies of the stronger 4f7/2 peaks areobserved at 83.3 eV in both Au@Pd and
Au@Pd/TiO2 catalysts. These peaks shift to alower value by 0.7 eV compared to that of pureAu (4f7/2 = 84.0 eV), as shown by a red dottedlines.17) Negative shifts of Au peaks in Au@Pd
and Au@Pd/TiO2 indicate that the electron
density around Au atom increases and thebinding energy decreases in comparison withthat of pure Au atoms due to an effect of thepresence of Pd shell. Similar negative chemical
shifts in Au@Pd and Au@Pd/TiO2 indicates thatthe presence of TiO2 catalytic support does notaffect the electronegativity of Au core.
In the range of 330–345 eV (Fig. 7b), Pd 3d3/2
and 3d5/2 peaks are observed. No peaks of PdO
are found indicating that oxidation of Pd shellis insignificant. The Pd 3d3/2 and 3d5/2 peaks in
Au@Pd/TiO2 are much sharper than those in
Au@Pd, indicating that energy distributions of
Pd states in Au@Pd/TiO2 are narrower thanthose in Au@Pd. The binding energy of thestronger Pd 3d5/2 peak in Au@Pd shifts to alower value of 335.1 eV by 0.2 eV, whereas thatin Au@Pd/TiO2 shifts to a further lower valueof 334.5 eV by 0.8 eV compared to that of purePd (3d5/2 = 335.3 eV), as shown by a red dottedline.17) It is known that work functions of Au(5.1 eV)18) and Pd (5.1 eV) are identical, andthey are larger than that of anatase-TiO2 (4.0eV). Therefore, the larger negative shift of Pd
peak in Au@Pd/TiO2 than that of Au@Pd is
explained by electron transfer from TiO2 to Pd
shells in Au@Pd/TiO2 particles.

3d5/2

335.3 eV334.5 eV335

330

Binding energy (eV)Fig. 7. XPS spectra of (a) Au and (b) Pd
components of Au@Pd and Au@Pd/TiO2
nanocatalysts. Standard peak positions areshown by dotted red lines.
Temporal variation of total gas (H2 + CO2)
generation by decomposition of FA using
Au@Pd and Au@Pd/TiO2 catalysts at 30, 60,
and 90 °C was measured (Fig. 8). With
increasing the reaction temperature from 30 oC
to 90 oC, the formation rate of H2 + CO2 gases
increases. The formation rate of H2 + CO2 gases
using Au@Pd/TiO2 catalysts is faster than thatusing bare Au@Pd in all the three temperatures,
although difference in the catalytic activity
decreases with increasing the reaction
temperature.
Table 1 shows the Rinitial values obtained
from equation (2) along with reported data for
Ag@Pd, Ag82Pd18@Pd, and Ag93Pd7@Pd/TiO2
catalysts.9,11) With increasing the reaction
temperature from 30 oC to 90 oC, the Rinitialvalues of Au@Pd and Au@Pd/TiO2 catalysts
increase by factors of 49 and 25, respectively.
The Rinitial values of Au@Pd/TiO2 are larger
than those of Au@Pd by factors of 1.53.0. The
Rinitial values of Au@Pd and Au@Pd/TiO2 aresmaller than those of Ag82Pd18@Pd and
Ag93Pd7@Pd/TiO2 by factors of 1.617 and令和元年度

九州大学大学院総合理工学報告

Total volume of H2 and CO2 / ml

10
8

(a)
30 oC

Au@Pd/TiO2

第 2 号

Tempe-

Catalyst

Rinitial

rature

2

Au@Pd
0

10

20

30

40

50

60

70

80

Reaction time / s
14
12

(b)
60 oC

Au@Pd/TiO2

10
8
6

Au@Pd

4
2
0

10

20

30

40

50

60

70

Au@Pd
Au@Pd
Au@Pd
Au@Pd/TiO2
Au@Pd/TiO2
Au@Pd/TiO2
Ag@Pd
Ag@Pd
Ag82Pd18@Pd
Ag82Pd18@Pd
Ag82Pd18@Pd
Ag93Pd7@Pd/TiO2
Ag93Pd7@Pd/TiO2
Ag93Pd7@Pd
/TiO2

30
60
90
30
60
90
20
35
27
60
90
27
60
90

Refs

(L / g h)

(○C)

4

0

27

Table 1. Initial hydrogen production rates from
catalytic decomposition of formic acid in water.

60Total volume of H2 and CO2 / ml

第 41 巻

0.043
0.12
2.1
0.13
0.21
3.2
3.67
4.58
0.71 ± 0.06
1.98 ± 0.18
3.34 ± 0.22
46.03 ±
2.27
143.77 ±3.33
371.21 ±10.54

This work
This work
This work
This work
This work
This work
Ref. 9Ref. 9Ref. 9Ref. 9Ref. 9Ref. 11
Ref. 11
Ref. 11

20

2

(c)
90 oC

Au@Pd/TiO2

1

15

0

5

0

Au@Pd/TiO2

Au@Pd

10

ln(Rinitial)

Total volume of H2 and CO2 / ml

Reaction time / s

-1
-2
Au@Pd

-3
0

2

4

6

8

10

Reaction time / s

Fig. 8. Gas generation by decomposition of
formic acid (0.25 M, 20 ml) versus time in thepresence of Au@Pd and Au@Pd/TiO2 catalysts
at (a) 30 oC, (b) 60 oC, and (c) 90 oC.
116685, respectively. These results indicatethat Au core is less active than AgPd alloy core
in M@Pd (M = metal or alloy) core-shellcatalysts.
The apparent activation energies were
estimated from the following relation using
Rinitial data given in Table 1.
ln(Rinitial) = −Ea/RT + C(4)

In this equation, Ea is the apparent activationenergy, and C is a constant. Fig. 9 shows plots

-4
2.7

2.8

2.9

3.0

3.1

3.2

3.3

3.4

1000 / T

Fig. 9. Plots of ln(Rinitial) vs 1000/T for
Au@Pd and Au@Pd/TiO2 catalysts.
of ln(Rinitial) vs 1000/T for Au@Pd and
Au@Pd/TiO2 catalysts. From slopes of each line,
the Ea values were estimated as 58 and 47 kJ
mol-1 for Au@Pd and Au@Pd/TiO2, respectively.
The lower Ea value obtained using Au@Pd/TiO2
than that using Au@Pd agrees with the higher
catalytic activity of the former catalyst.
The work function has been regarded as animportant parameter of the catalytic system for
the FA decomposition system over M@Pd (M =Ag, Rh, Au, Ru, Pt) core–shell catalysts. The
catalytic activity increases concomitantly with

28

Hydrogen Production from Formic Acid Using Au@Pd and Au@Pd/TiO2 Nanocatalysts

decrease in the work function.9) The current
results show that the hydrogen production rateis greatly enhanced in the presence of TiO2 for
the Au@Pd system, as observed for AgPd@Pdcatalyst.10-13) Larger negative chemical shift(0.8 eV) was observed for the XPS peak of Pd(3d5/2) shell for Au@Pd/TiO2 compared with thatfor Au@Pd (0.2 eV), because the work functionof TiO2 (4.0 eV) is lower than Pd (5.1 eV) andAu (5.1 eV). This implies that electrons aretransferred from TiO2 to Pd shells in thepresence of TiO2. Such electron transfer fromthe TiO2 support to Pd shells causes the higher
catalytic activity of Au@Pd/TiO2 than that of
Au@Pd. When the negative peak shift of Pd(3d5/2) in Au@Pd/TiO2 (0.8 eV) was comparedwith that of AgPd@Pd/TiO2 (2 eV)11), it issmaller by 1.2 eV. This indicates that a smaller
amount of electrons is transferred from TiO2 to
Pd shells in Au@Pd/TiO2 catalyst. This is amain reason why the catalytic activity of
Au@Pd/TiO2
is
lower
than
that
of
AgPd@Pd/TiO2.
The current results show that the apparent
activation energy of reaction (1) using
Au@Pd/TiO2 catalyst decreases by 19% in thepresence of TiO2. Based on previous density
functional theory calculations on intermediate
species,19,20) formate
isanimportantintermediate in FA decomposition on metallic
catalysts. A possible decomposition scheme offormate on Au@Pd/TiO2 is shown in Fig. 10.
Decomposition starts by activating the C–Hbond of formate adsorbed on the catalytic
surface. Under our conditions, monodentate
formate (species A) transforms eﬃciently tomore stable bidentate formate (species B), in
which both oxygen atoms bind to the catalyst
surface. Otherwise CO is formed throughdecomposition of monodentate formate (species
A).9) Bidentate formate (species B) decomposes
into CO2* + H* (species C), where X* denotes
intermediates adsorbed onto the surface.
Recombination of two H* and CO2* leavingfrom the surface results in the formation of CO2+ H2 gases (species D). Based on theoretical
calculations on potential energy surfaces(PESs) of the FA decomposition reaction, theenergy barrier from species B to species C ishighest. It is the rate-determining step. The
greater degree of electron transfer from the Pdlayer to intermediate species A–C in thepresence of TiO2 strengthens the adsorption offormates and decreases the energy barriers ofhydrogen formation and proton diﬀusion overcatalysts.

4.

Using an MW-heating method, TiO2supported Au@Pd core–shell nanoparticles
with an average diameter of 152 nm wereprepared. To examine the eﬀects of TiO2, bare
Au@Pd were synthesized under MW heating in
the absence of TiO2. The initial hydrogenformation rate of Au@Pd/TiO2 from FA, 0.13
L g-1 h-1, was 3.0 times higher than that of bare
Au@Pd at 30 oC. The apparent activationenergy of dehydrogenation of FA by Au@Pd
catalysts decreased from 58 kJ mol-1 to 47 kJ
mol-1 in the presence of the TiO2 support. When
XPS spectra of the catalysts were observed,
larger negative chemical shifts were observedfor Pd peaks of Au@Pd/TiO2 in comparison withthose of bare Au@Pd because of electrondonating eﬀects of TiO2 to Pd shells. Based on
the findings presented above, the marked
enhancementofcatalytic
activityof
Au@Pd/TiO2 was attributed to electron transferfrom TiO2 to Au@Pd catalysts, promoting C–Hcleavage of FA over the surface of Au@Pd/TiO2
nanocatalysts (species B  C in Fig. 10). In fact,
CO emission, which reduces catalytic activity of
Au@Pd/TiO2, was not observed throughdecomposition of monodentate formate (species
A in Fig. 10) at 30–90 oC.
Acknowledgments
This work was supported by JSPS KAKENHI
Grant number 25286003.
References
1)
2)

H

H

(C)
O C O

(Ａ)

(B)

C O
O
H

O

Au@Pd

Au@Pd

Au@Pd

TiO2

TiO2

TiO2

C

O H

(D)

3)

H H
CO2 + H2

Fig. 10. The schematic diagram of formic aciddecomposition pathways on the Pd surface of
Au@Pd/TiO2.

Summary and Conclusion

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